The invention relates generally to optical sensors for determining fluid levels. More particularly, the invention relates to point level sensors of the type that use prisms for reflecting light internally to indicate fluid level.
Optical fluid point level sensors are well known. Such sensors commonly use a prism having a conical tip. The prism is transparent to a beam of light launched into the prism from a light source, such as an LED. The light travels through the prism towards a surface of the conical tip and impinges on the surface at a determinable angle of incidence. The prism is made of a material that has a refractive index such that there exists a critical angle of incidence at which light is internally reflected to another surface and back to a photodetector, such as a phototransistor. Whether such internal reflection occurs depends on the refractive index of the fluid to which the conical tip is exposed and the angle of incidence. The critical angle is defined by the following equation: EQU .theta..sub.c =sin.sup.-1 (n.sub.2 /n.sub.1) Eq. 1
where n.sub.2 is the index of refraction of the fluid, and n.sub.1 is the index of refraction of the prism conical tip. Thus, for air, n.sub.2 =1.00 and for glass, n.sub.1 =1.50. Accordingly, for total internal reflection the critical angle with respect to an air/glass interface is 42.degree.. By comparison, if the conical tip is exposed to water as the fluid, the refractive index of water is 1.33. Thus the critical angle for total internal reflection with respect to a glass/water interface is about 62.5.degree..
By forming the conical surfaces such that the light transmitted therethrough is incident at 45.degree., the light will undergo total internal reflection (hereinafter "TIR") when the conical tip is exposed to air (because 45.degree. is greater than the critical angle of 42.degree. for a glass/air interface), but will not undergo TIR when the conical tip is exposed to water (because 45.degree. is less than the critical angle of 62.5.degree. for a glass/water interface). By positioning a light detector to receive the light that is internally reflected, the prism can be used as a point level detector for the water level. The transmitted light that is not internally reflected is refracted into the fluid, as is well known.
Note that for TIR to occur, the refractive index of the conical tip must be higher than the refractive index of all fluids which are to be detected (in this example, air and water).
Such a prismatic sensor can also be used to detect an aircraft fuel/air interface when the prism material is made of a higher refractive index such as 1.65, because the index of refraction for fuel is on the order of 1.4 to 1.5. Thus, it is known to use such sensors for fuel level detection by detecting the ullage/fuel interface at different levels in a fuel tank.
Conventional optical point level sensors typically include the light source and detector in close proximity to the prism. The light sources and detectors, however, are commonly solid state devices that are sensitive to temperature variations. Therefore, using the source and detector near the prism may expose the solid state devices to adverse environmental conditions in some applications.
To reduce the environmental effects, it is also known to couple light to and from the prism with an optic fiber. This allows the photosensitive devices to be located remote from the prism in a less hostile environment, such as an electronics bay, for example.
All of the aforementioned known designs suffer from the problem that the light launched into the prism tends to scatter and spread, such that even under conditions of TIR, very little light is returned to the detector. Although lenses have been proposed to collimate the light into the prism, such as discussed in U.S. Pat. No. 5,159 834 issued to Eisele, such designs do not provide multichannel operation or require the use of expensive components per channel.
The objectives exist, therefore, to provide an optical fluid point level sensor that produces a higher intensity optical output than known heretofore, and further makes available multichannel operation and redundancy. Such a sensor preferably will use a reduced number of optical components within a reduced envelope so as to reduce sensor cost and simplify manufacture and use.